The invention relates to a support means for a tread of a tire. The support means constitutes, with said tire and its mounting rim, a rolling assembly for a vehicle, which is intended to be able to travel after a consequent, unexpected loss of pressure from the tire, the tire being more particularly a tubeless tire of the heavy-vehicle type, the form ratio of which is at most 0.8.
French Patent 2,135,333 describes, as a support means which makes it possible to reduce the risks due to deflation or punctures to a minimum, an inflatable toroidal reservoir comprising means for restricting expansion during normal operation of the tire, and means for releasing said restriction means and permitting expansion. The restriction means are formed of a flexible material which is folded and secured so as to form a holding band, whereas the means for releasing the restriction means may comprise either a seam or an adhesive connection, or a fixing element made of fabric.
French Application FR 96/14631, which has not been published to date, describes and claims, as support means for a tread, a toric membrane of reinforced rubber, inflated to a pressure p0 greater than the pressure p1 of the tire cavity, and having, in the inflated state, a crown radius RM which is less than the loaded radius of the tire used at its recommended pressure, said membrane, being reinforced, at least in its crown, by a crown reinforcement formed of at least one ply of cords or cables, said crown of said membrane furthermore comprising a hooping reinforcement composed of at least one ply of cords or cables oriented circumferentially and having a breaking load per cm of ply at least equal to the product of the crown radius RM times the pressure, per cm2 of surface area of said ply, resulting in a tension per cm of ply equivalent to the tension due to the maximum centrifugal force to which the tire can be subjected and permitting breaking of the hooping cords or cables for a pressure difference p0xe2x88x92p1xe2x80x2, existing in the case of the tire undergoing a loss of pressure, greater than the initial pressure difference p0xe2x88x92p1, that is to say, during normal travel.
The toric membrane may be closed or open. It is said to be closed when its cross-section has a continuous contour, that is to say, one which resembles a section of an inflated inner tube; it is said to be open if the contour of its cross-section is discontinuous, for example when the sidewalls of said membrane have ends located at the level of the beads of the tire in which said membrane is inserted, and being integral therewith or independent therefrom.
The internal pressure p0 of said membrane, when measured cold, that is to say, at 20xc2x0 C., is greater than the pressure p1 of the inner cavity of the tire by an amount of between 0.5xc3x97105 Pa and 5.0xc3x97105 Pa, depending on the dimensions of the tire in question. Given that the crown radius RM of the toric membrane is preferably between 0.80 and 0.97 times the loaded radius of the tire, mainly for reasons of heating of said tire, an excessively large pressure difference risks adversely affecting a certain number of properties of the tire itself, for example the life of the carcass reinforcement of said tire, while demanding an excessively large hooping reinforcement.
The crown reinforcement is preferably formed of two plies of cords or cables which are parallel to each other within each ply and are crossed from one ply to the next, forming an angle of between 50xc2x0 and 85xc2x0 with the circumferential direction. The cables or cords are advantageously textile ones for reasons of lightness, flexibility and good corrosion resistance, and preferably are made of aromatic polyamide. The axial ends of the two plies are preferably located on the sidewalls of the membrane, such that, if the maximum axial width of the carcass reinforcement of the tire is referred to as S, the width of the plies is preferably between S and 1.30 S.
The toric membrane, according to the invention described in the above French application, may possibly comprise sidewalls each reinforced by at least one ply of radial cords or cables, said sidewalls possibly being provided advantageously with radial grooves which open onto the metal mounting rim of the tire.
Under normal conditions of travel of the assembly formed by the tire, its mounting rim and the membrane, load, pressure and speed conditions recommended for the tire in question, the membrane retains an equatorial radius which is practically constant and less than the loaded radius of the tire, and the outer walls of its sidewalls are for their very major part in permanent contact with the inner walls of the tire. The friction between said sidewalls causes premature degradation and wear of the impermeable rubber layer which covers the inner sidewall of the tire.
In order to overcome these disadvantages, the invention proposes an original structure of the toric supporting membrane which makes it possible to control its meridian profile under normal conditions of travel, while permitting complete expansion thereof in the event of a loss of pressure from the cavity of the tire.
The toric membrane of reinforced rubber according to the invention which is used as a support means for the tread of a tire and forms with said tire and its mounting rim, the nominal diameter of which is DS and the flange of which has an outer diameter DR, a rolling assembly which can roll when the tire is subject to a loss of pressure, inflated to a pressure p0 greater than the pressure p1 of the cavity of the tire, and having in the inflated state a crown radius RM less than the loaded radius RE of the tire when used at its recommended pressure, said membrane being reinforced in its crown by at least two plies of cords or cables which are parallel to each other within each ply and are crossed from one ply to the next, said crown of said membrane furthermore comprising a hooping reinforcement composed of at least one layer of cords or cables which are oriented circumferentially and have a breaking load per cm of ply which makes it possible to resist the tension due to the maximum centrifugal force to which the tire is subject, increased by the tension due to the pressure difference p0xe2x88x92p1 existing during normal travel, but to break for a greater pressure difference p0xe2x88x92pxe2x80x21, having each of its sidewalls reinforced by at least one ply, is characterized in that, viewed in meridian section, the sidewall reinforcement ply is wound in each bottom part of the sidewall around an annular reinforcement element, the internal diameter of which lies between a value equal to DR and a value equal to DS and the constitution and transverse dimensions of which permit the breaking thereof after that of the hooping reinforcement, and for a pressure difference p0xe2x88x92pxe2x80x21, existing in the case of the tire undergoing a loss of pressure, greater than the initial pressure difference p0xe2x88x92p1, said ply not being integral with said annular element, and having within each of the sidewalls a meridian length such that its meridian profile in the inflated state permits the outer sidewall of the membrane not to be in contact with the inner sidewall of the tire radially outside a position defined by the diameter DS+2(DRxe2x88x92DS).
Advantageously, the breaking load of the reinforcement element around which the sidewall reinforcement ply is wound is between 1.2 and 4 times the breaking load per cm of the hooping reinforcement used in the supporting membrane.
The sidewall reinforcement ply may be radial, or formed of cords or cables which form with the circumferential direction an angle which may for example be between 50xc2x0 and 90xc2x0. Preferably, for manufacturing reasons, the crown reinforcement of the membrane and its sidewall reinforcement plies will be formed by the same plies of oblique cords or cables: a first ply, one of the ends of which is located in the region of one of the shoulders of the membrane, and the other end of which is the end of its upturn around the reinforcement element located on the side opposite said shoulder; a second ply, one of the ends of which is located in the region of the other shoulder of the membrane, and the other end of which is the other end of the upturn. Each individual ply may also be formed of cords or cables which are radial in the sidewall parts and oblique in the crown part of the membrane.
The annular reinforcement element may advantageously be a single cable of aromatic polyamide formed of several strands. It may also be formed by the winding of a filament of aromatic polyamide on itself so as to form a section of ply which is virtually perpendicular to the axis of rotation of the assembly.
The support strip may have elements in relief which are substantially frustoconical and which form circumferential rows of blocks or pads, the hooping reinforcement of the membrane preferably being located on the radially outer face of the support strip and being in the form of a plurality of narrow strips of several circumferential textile cables of widths substantially equal to the widths of the cutouts or grooves between rows of elements in relief of the support strip and arranged in said grooves independently of the support strip. This hooping structure, when the inner cavity of the tire is devoid of inflation gas and by breaking of the circumferential cables of the strips due to the significantly increased pressure difference, permits faster, more complete expansion of the toric membrane.